A PRACTICAL APPROACH TO PREDICTING SUB-GRADE MODULI USING FWD

1. A PRACTICAL APPROACH TO PREDICTING SUB-GRADE MODULI USING FWD Abdenour Nazef

2. INTRODUCTION Importance of sub-grade modulus in design & analysis of pavements current use of deflection-based techniques Deflections non-destructively induced & measured

3. INTRODUCTION (Cont.) 2 types of commercial devices commonly used: Vibratory devices: steady-state sinusoidal load applied (ex: Dynaflect) Impulse/falling weight devices: impulse load applied (ex: FWD)

4. Vibratory Wave Form Plot

5. Pulse Loading Form Plot

6. INTRODUCTION (Cont.) FWD Loading: Load generated by a dropping a mass from a specified height Loading simulates actual wheel loads Deflections measured by velocity tranducers

7. Schematic of a Falling Mass System

8. INTRODUCTION (Cont.) FWD (cont.): Major advantage of impulse loading device is its ability to more accurately simulate a moving wheel load in both magnitude and duration of loading

9. FWD Device

10. INTRODUCTION (Cont.) Dynaflect Loading: Static load (trailer weight) of 2000 lb. applied through a pair of rigid steel wheel Dynamic load (1000 lb peak-to-peak) generated using 2 counter-rotating steel weights with a falling weight system Dynamic load superimposed on static load

11. Dynaflect Device

12. Dynaflect Loading System

13. INTRODUCTION (Cont.) Dynaflect (cont.): Major limitations are fixed magnitude & frequency of loading

14. OBJECTIVES Assess feasibility of using FWD-induced deflections in FDOT current procedure for determining sub-grade moduli Recommend a practical approach for using FWD data that would also ensure compatibility with that of Dynaflect

15. TESTING PROGRAM 302 test sections from interstate system Each section is 1-mile long 14 test locations randomly selected within each section along outer wheel path At each location, testing completed concurrently with both devices

16. TESTING PROGRAM (con�t) FWD Sensors configuration

17. TESTING PROGRAM (con�t) Dynaflect Sensor configuration

18. DATA ANALYSIS Moduli Prediction: Current Method (Dynaflect data): logEr = 4.0419 � 0.5523?logd4 Er = Sub-grade modulus, in psi; and d4 = Deflection measured at 36 in., in mils.

19. DATA ANALYSIS (con�t) Moduli Prediction (cont.): Proposed Method (FWD data): Er = 0.24P /dr?r P = Applied load; dr = Deflection at a distance r; and r = Distance at which the deflection is measured.

20. DATA ANALYSIS (con�t) Data Range : All Data

21. DATA ANALYSIS (con�t) Data Range : Data<32000 psi

22. DATA ANALYSIS (con�t) Adjusted FWD Data by 1.2 factor

23. DATA ANALYSIS (con�t) Higher level of agreement obtained if AASHTO equation adjusted as follows: EFWD = 3.3863?(EAASHTO)0.898 EFWD = 0.03764?(P /dr)0.898

24. DATA ANALYSIS (con�t) E based on proposed prediction equation

25. CONCLUSIONS A strong correlation, of the form y = ??x? with an R-square value of 0.88, obtained between AASHTO equation and current Dynaflect-based method Within the same test site, AASHTO equation would generally result in lower E values when E<40,000 psi. Above the 40,000-psi mark, the reverse would be obtained

26. CONCLUSIONS (cont.) For E<32,000 psi, E (current method) 1.2 times higher that E (AASHTO Eq.) Once E (AASHTO Eq.) adjusted using a 1.2 multiplier, level of agreement increased. A higher agreement obtained when adjustment is 3.3863?(EAASHTO)0.898

27. RECOMMENDATION The following simple power law equation [Er = 0.03764?(P /dr)0.898] appears to result in a higher level of agreement with the current method. It is therefore recommended that this approach be implemented to predict the embankment moduli of in-service pavements to ensure a better compatibility with data collected with the Dynaflect device.